Many vehicles having hydrostatic transmissions also are provided with an underspeed actuator which functions to reduce the load on the engine by reducing the displacement of the pump of the hydrostatic transmission when the speed of the engine falls below a preselected speed range in response to an external load such as when the vehicle is operated uphill or otherwise encounters an increased load. One of the problems encountered with such underspeed actuators is that of providing rapid response thereof when a load is suddenly imposed on the hydrostatic transmission causing a sudden increase in the pump output pressure and a decrease in engine speed. It is desirable to provide means for rapid response of the underspeed acutator under such conditions to enable the engine to maintain a higher operating speed thereby improving performance and efficiency.